EP0092461B1 - Annular closure slab for a fast neutron nuclear reactor vessel - Google Patents

Description

The invention relates to an annular closure slab for the vessel of a fast neutron nuclear reactor.

Fast neutron nuclear reactors generally have a concrete structure comprising a cylindrical well shaft with a vertical axis inside which is placed the main reactor vessel surrounded by the safety vessel and closed by the slab which rests on the upper part from the tank well. The main tank and the reactor safety tank are suspended from the lower part of the slab which is made up of a composite structure of steel and concrete.

This structure is constituted by an annular envelope filled with concrete leaving at its central part a cylindrical space to allow the slab to receive the large rotating plug carrying all the devices for handling the fuel assemblies constituting the core of the reactor arranged at the inside of the tank.

The annular envelope is constituted by two coaxial cylindrical ferrules and by two annular flanges connected to each of the ferrules at their upper part and at their lower part, respectively.

Cylindrical locations are provided inside the envelope by ferrules passing through the slab over its entire thickness and emerging at the level of circular openings provided in the upper and lower flanges of the slab. The concrete filling occupies the interior volume of the envelope with the exception of these cylindrical spaces allowing the passage of the components of the reactor plunging through their lower part into the tank, such as the primary pumps and the intermediate exchangers.

Radial direction stiffeners allow rigid assembly of the cylindrical ferrules, upper and lower flanges and ferrules for passage of the components. These various elements are interconnected by welding.

The external cylindrical shell of the cover of the slab is used to fix the latter above the tank well. A ferrule of the same diameter is in fact fixed to the upper part of the vessel well and the connection of the external shell of the slab on this support ferrule allows the slab to be fixed.

The reactor vessel is intended to contain the primary fluid maintained at high temperature by the heat released by the core. This primary fluid is generally liquid sodium.

The parts of the slab directed towards the interior of the tank, that is to say the bottom sole and the sheaths for the passage of the components, must therefore be cooled to limit their heating and to avoid a reduction in their mechanical resistance. Cooling tubes are therefore placed in contact with the bottom flange and the sheaths for the passage of components, inside which a circulation of water is established, these cooling tubes being embedded in the concrete filling the slab.

In addition, the bottom sole is protected from the heat of the primary fluid by an insulation.

The dilations of the slab are therefore limited and it is thus possible to connect it to the support ferrule secured to the vessel well.

When the large rotating plug rests in its housing in the center of the annular slab, its extremely high weight is entirely supported by the slab, the upper sole of which works in compression and the lower sole in tension.

The entire metal structure of the slab comprising the outer shell, the inner shell, the upper and lower flanges, the sheaths for the passage of the components and the vertical stiffeners of radial direction must therefore have very high strength and very high rigidity. .

In particular, to transmit the shearing forces without discontinuity, the stiffeners must be very close together, so that the internal volume of the slab envelope is extremely fragmented. The filling concrete therefore practically does not participate in the resistance of the assembly and only serves as protection against radiation of the materials contained inside the tank.

The metal structure of the slab is extremely heavy, expensive and difficult to weld, this structure being made even more complex by the fact that it is necessary to provide cooling tubes in contact with the sheaths for passage of the components and in contact with the bottom flange. which supports very important constraints. These cooling pipes must pass through the radial stiffeners in which numerous openings must be provided.

The object of the invention is therefore to propose an annular closure slab for the vessel of a fast neutron nuclear reactor, the concrete structure of which comprises a cylindrical vessel well with a vertical axis on the upper part of which the slab constituted horizontally rests. by a composite steel and concrete structure which itself carries the tank suspended from its lower part and which comprises an annular envelope constituted by two coaxial cylindrical ferrules and two annular flanges connected to each other directly and by means of an assembly stiffening, filled with concrete with the exception of cylindrical spaces with axes parallel to the axis of the ferrules and passing through the slab over its entire thickness for the passage of the components of the reactor plunging into the tank, this slab having great resistance and great rigidity despite a simple structure, greater ease of production and less mass.

For this purpose, the slab comprises, inside the envelope, as a stiffening element, at least one frustoconical ferrule coaxial with the ferrules limiting the slab connected by welding to the envelope along its small base, in the vicinity of the upper part of this envelope and along its large base, in the vicinity of the lower part thereof, pierced with openings for the passage of components and dividing the internal volume of the envelope into two superimposed parts.

According to a preferred embodiment of the invention, the upper part of the internal volume of the envelope, above the frustoconical shell, is filled with concrete incorporating metal reinforcements for reinforcement and constituting a continuous resistant structure over the entire periphery of the slab.

• La fig. 1 représente, dans une vue en coupe par un plan vertical, suivant AA de la fig. 2, une dalle suivant l'art antérieur en position sur un réacteur nucléaire à neutrons rapides.
• La fig. 2 représente une demi-vue de dessus de la dalle représentée à la fig. 1.
• La fig. 3 représente une vue en coupe, suivant BB de la fig. 4, d'une dalle suivant l'invention en position sur le puits de cuve d'un réacteur nu'-' cléaire à neutrons rapides.
• La fig. 4 représente une demi-vue de dessus de la dalle représentée à la fig. 3.

In order to clearly understand the invention, we will now describe, by way of nonlimiting examples, with reference to the attached figures, an embodiment of a slab according to the prior art and an embodiment of a slab according to the invention.

• Fig. 1 shows, in a sectional view through a vertical plane, along AA in FIG. 2, a slab according to the prior art in position on a fast neutron nuclear reactor.
• Fig. 2 shows a half top view of the slab shown in FIG. 1.
• Fig. 3 shows a sectional view, along BB of FIG. 4, of a slab according to the invention in position on the vessel well of a nu'- 'key fast neutron reactor.
• Fig. 4 shows a half top view of the slab shown in FIG. 3.

In fig. 1 we see the tank well 1 of a fast neutron nuclear reactor comprising at its upper part an anchoring device 2 of a support shell 4 whose diameter corresponds to the inside diameter of the tank well and to the outside diameter of slab 3.

As can be seen in FIGS. 1 and 2, the slab comprises an outer ferrule 5 of the same diameter as the support ferrule 4 on which it is fixed by welding and an internal ferrule 7 coaxial with the ferrule 5. These ferrules 5 and 7 are connected by welding to a sole upper annular 8 and a lower annular sole 9, all of the ferrules 5 and 7 and the soles 8 and 9 constituting the envelope of the slab.

The cylindrical ferrules and the flanges are also connected to each other by vertical stiffeners 10 in the radial direction.

The internal volume of the envelope is filled with concrete 14 with the exception of vertical cylindrical spaces 12 formed by cylindrical sleeves 15 inside the envelope of the slab. These cylindrical spaces 12 crossing the entire thickness of the slab allow the passage of the components of the nuclear reactor immersed in the tank, such as pumps or intermediate exchangers. The sleeves 15 are connected by welding to the flanges 8 and 9 which have circular openings corresponding to these sleeves and to the outer shell 5, by means of vertical stiffeners of radial direction 11.

The set of stiffeners 10, sleeves 15 and stiffeners 11 ensures the stiffening of the envelope of the slab.

It can be seen that the internal volume of this slab is highly fragmented and that the concrete 14 cannot thus constitute a resistant structure over the entire periphery of the slab.

On the other hand, the cooling tubes 6 of the sleeves 15 and of the lower flange 9 must pass through numerous vertical stiffeners during their journey.

The main tank 16 and the safety tank 17 of the reactor are constituted by a cylindrical shell of large diameter whose upper part visible in FIG. 1 is fixed inside the slab for the suspension of these tanks and by a curved bottom comprising several elements of spherical shape.

The lower part of the cylindrical shell and the domed bottoms of the tanks have not been shown in FIG. 1 inside the tank well.

In fig. 3 shows a slab according to the invention 20 resting on the upper part of the tank well 21.

As can be seen in FIGS. 3 and 4, this slab 20 comprises an envelope constituted by an outer ferrule 25 and an inner ferrule 27 coaxial, an upper sole 28 and a lower sole 29. Sheaths for the passage of the components 24 are disposed inside this envelope at the level of openings 22 provided in the upper and lower soles of the envelope.

The stiffening assembly of the envelope is constituted by a simple frustoconical ferrule 30 coaxial with the ferrules 25 and 27.

The frustoconical shell 30 is connected by welding along its small base to the internal cylindrical shell 27, in the vicinity of the upper part of the slab. The ferrule 30 is also connected by welding along its large base to the outer ferrule 25, in the vicinity of the lower part of the slab.

The ferrule 30 is pierced with openings allowing the passage of the sleeves 24 and welded to these sleeves 24 along the outline of these openings.

The frustoconical ferrule 30 separates the internal volume of the envelope of the slab into two superimposed parts, one located below the ferrule 30 and the other above this ferrule. The part situated below the shell 30 is filled with radiological protection concrete 34 thanks to filling openings passing through the shell 30.

The upper part of the internal volume of the envelope, above the frustoconical shell 30, is filled with concrete incorporating metal reinforcements 33 arranged in particular along concentric paths throughout the internal volume of the slab. In this way, the reinforced concrete filling this part of the slab plays an important role in the resistance of the latter.

The outer shell 25 is reinforced at its lower part 35 where the large base of the frustoconical shell 30 is connected.

On the outer surface of this reinforced part of the shell 25 is fixed a support structure 37 constituted by an annular belt with a square section. This annular structure 37 consists of a steel casing welded to the outer ring 25 in its reinforced part, filled with concrete. The support structure 37 rests on a sliding support 38 carried by the upper part of the tank well 21.

The main tank 40 and the safety tank 39 are fixed by their upper part to the frustoconical shell 30, so that they are suspended under the slab 20.

When the slab is loaded at its central part, in the well formed by the ferrule 27 where the large rotating plug is placed, the load due to this rotating plug and to the devices for handling the fuel assemblies that it supports, the distribution stresses in the slab is very different from the distribution in the case of a slab according to the prior art as shown in FIG. 1. Indeed, the weight of the large rotating plug and of the components carried by the slab translates into compression forces exerted on the upper sole of the slab and on the upper part of the frustoconical shell whereas the traction forces acting on the lower part of the frustoconical shell are taken up by the reinforced part of the external shell 25 and by the support structure 37.

On the other hand, the traction forces exerted on the lower sole of the slab are of very low amplitude.

The part of the slab located below the frustoconical shell 30 undergoes very low stresses and in this zone the internal volume of the envelope of the slab is filled with radiological protective concrete.

On the other hand, the part of the internal volume of the envelope of the slab situated above the frusto-conical shell 30 is filled with concrete incorporating metal reinforcements arranged in particular concentrically in the slab.

This reinforced concrete ensures the mechanical strength of the slab at the same time as the ferrules, the soles, the sheaths and the frustoconical ferrule serving as a stiffener.

It is therefore possible to reduce the thickness of the metal elements constituting the envelope of the slab and in particular to reduce the thickness of the lower sole which is only subjected to stresses of small amplitude.

The frustoconical shell 30 which is embedded in the protective concrete cannot be deformed by buckling so that it is possible to use for its manufacture a sheet of moderate thickness and which can be avoided to reinforce it.

Reinforcing the outer shell 25 in its. lower part and surround this ferrule by the support structure 37 allows to take up the radial forces and to have the neutral line of the deformations of the slab as close as possible to the supports.

Finally, the fact of welding the sleeves 24 to the frusto-conical ferrule 30, the latter being itself fixed to the external and internal viroies 25 and 27, makes it possible to take up part of the shearing forces.

The main advantages of the slab according to the invention are that it makes it possible to obtain great mechanical strength and great rigidity thereof with a much simpler structure, easier to produce and of less weight.

On the other hand, the new structure of the slab according to the invention makes it possible to rest it on the upper part of the tank well and to avoid connecting it to a support ferrule secured to the tank well, by a solder exposed to the heat of the primary fluid.

Part of the concrete filling the slab contributes to obtaining good mechanical strength and great rigidity thereof.

The fact of avoiding the use of vertical stiffeners of radial direction over the entire height of the slab makes it possible to arrange the cooling tubes of the lower flange and of the sheaths for passage of the components easily, without having to provide for perforations in the stiffeners .

The invention is not limited to the embodiment which has just been described; this is how the frusto-conical shell can be welded at its ends on the upper and lower flanges instead of being welded on the external and internal ferrules.

The frustoconical ferrule can be welded at one of its ends on one of the flanges and at its other end on one of the ferrules.

It is also possible to use, in place of a single frusto-conical ferrule, a set consisting of at least two coaxial frusto-conical ferrules separated by a constant distance welded to the ferrules or to the flanges.

One can also imagine a stiffener assembly consisting of at least two coaxial frustoconical ferrules and separated by a constant distance, that is to say having the same angle at the top, and connected together by spacers. One can also imagine one or more frusto-conical rotors having reinforcements to prevent their buckling, although the concrete surrounding these stiffening ferrules practically prevents their deformation.

Finally, the slab according to the invention applies to all fast neutron nuclear reactors comprising an annular slab whose central part supports a heavy assembly such as a rotating plug for handling fuel assemblies.

Claims (8)

1. Annular closure slab for the vessel of a fast- neutron nuclear reactor the concrete structure of which incorporates a cylindrical vessel well (21) with a vertical axis on the upper part of which rests horizontally the slab (20) consisting of a composite structure made of steel and concrete which itself carries the vessel (40) suspended from its lower part and which comprises an annular enclosure consisting of two coaxial cylindrical shells (25, 27) and two annular sole-plates (28, 29) joined together directly and through the inter- mediacy of a stiffening assembly, filled with concrete except for cylindrical spaces (24) with axes parallel to the axis of the shells (25, 27) and passing through the slab (20) over its entire thickness, for the passage of the reactor components immersed in the vessel (40), characterised in that it comprises, inside the enclosure, as a stiffening member, at least one frustoconical shell (30) coaxial with the shells (25, 27) bounding the slab (20) and connected by welding to the enclosure along its small base, in the vicinity of the upper part of this enclosure and along its large base, in the vicinity of the lower part of the latter, pierced with openings (22) for the passage of the components and dividing the inner volume of the casing into two superposed parts.

2. Closure slab according to Claim 1, characterised in that the upper part of the enclosure above the frustoconical shell (30) is filled with concrete incorporating metal strengthening frames (33) and forming a continuous strong structure over the whole periphery of the slab (20).

3. Closure slab according to either of Claims 1 and 2, characterised in that the frustoconical shell (30) is fixed at its lower part on the lower surface of the outer shell (25) of the enclosure, in a part (35) of the latter which is strengthened and surrounded externally by a strengthening and supporting structure (37) enabling the slab (20) to rest on the upper part of the vessel well (21).

4. Closure slab according to any one of Claims 1, 2 and 3, modified in that the stiffening member (30) consists of at least two frustoconical shells with the same angle which are arranged coaxially with a constant spacing.

5. Closure slab according to Claim 4, characterised in that braces or reinforcements are arranged between the frustoconical shells.

6. Closure slab according to any one of Claims 1, 2 and 3, characterised in that the frustoconical shell (30) is welded at its lower end to the outer shell (25) of the enclosure and at its upper end to the inner shell (27) of the latter.

7. Closure slab according to any one of Claims 1, 2 and 3, characterised in that the frustoconical shell (30) is welded at its lower end to the lower sole plate (20) and at its upper end to the upper sole plate (28) of the enclosure.

8. Closure slab according to any one of Claims 1, 2 and 3, characterised in that the frustoconical shell is welded at one of its ends to one of the cylindrical shells (25, 27) of the enclosure and at its other end to one of the sole plates (28, 29) of this enclosure.

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